Hi,
no... i'm not starting off the TBF discussion again here.
But I am interested in first signs of the problems with the thrust bearing.
What are the first signs and symptoms?
- measurable wear at the crank?
- sound or noise?
- vibrations?
- performance?
- temperature of the engine?
- oil burn (smell)?
- oil/filter contains particles?
- ....
Can the bearing be repaired/replaced?
- when the engine is still running? (not turning)
- when the engine has seized?
- .....
Any real life experiences?
Thanks,
--
Theo
1988 928s4 cherry red
The Netherlands
http://928s4.no-ip.info
=======
On the Rennlist web message board there was a recent case reported where
the symptoms were stalling and slow engine turnover when trying to restart when
the engine was warm. High flex plate load and crank end float were
measured.
On engine disassembly, the thrust bearing face was still largely intact and
the crank weights had not made contact with the block, but the thrust
bearing had spun and ate at the crank journal and bearing saddle and cracked
the block. The thrust bearing has no tabs to prevent it from spinning and
relies on a tight fit to the block, but, as Jon mentioned, excessive load
against the bearing can break through the lubricant film and spin the bearing.
Bill
79
89 S4
=======
Theo,
The only real first sign you could see is particles in the oil filter, probably
accompanied with out of spec endplay on the crank.
All the other things you list are the near terminal signs that occur when the
white metal on the Thrust bearing has all gone, the thrust surfaces are now
steel/steel, the tolerance between the thrust surfaces is too high to maintain
the oil pressure generated by the shear of the oil between the faces (I forget
the technical term for this but the pressure generated is a lot higher than the
5 Bar from the pump) , the thrust bearing is spinning in the block and slowly
machining its way forward thro' the block. Probably accompanied with a bit of
vibration which may or may not be noticeable.
The terminal signs are failure to idle or start when hot along with vibration
..... This is when the crank has moved far enough forwards for the balance
weights to start hitting the block, and sometimes all the endplay in the little
end bearings (gudgeon pin in piston) has been used up so that the rods are
trying to push the piston/s sideways.
In short - releasing the clamp on the drive shaft and checking endplay is
probably the only way of reliably ascertaining whether TBF is happening in the
early stages.
I hope for you that this was a theoretical question?
Jon in NZ
=========
Jon,
thanks for the quick reply! So:
What are the first signs and symptoms?
- measurable wear at the crank (over 0.4mm play)
- oil/filter contains particles
Progressed symptoms:
- sound or noise? How to determine?
- vibrations? Probably?
- performance?
- temperature of the engine?
- oil burn (smell)?
- oil/filter contains particles
- ....
Terminal stage:
- failing to idle
- failing to run when warm
- grinding sound from within the engine
- oil/filter contains particles
Can the bearing be repaired/replaced?
- when the engine is still running? (not turning)
- when the engine has seized?
- .....
Any other comments or experiences?
Hope it remains a theoretical question ;-)
--
Theo
1988 928s4 cherry red
The Netherlands
http://928s4.no-ip.info
=====
Theo
looking at the specs - max end float when new is 0.3mm, max worn is 0.4mm so if
the end float is greater than 0.4mm I would say that you have to pull the engine
(Realistically you can't do the job with the engine in the car) and replace the
thrust bearing.
Realistically if there are white metal particles in the filter you have to pull
the engine, as there is some form of bottom end bearing wear.
If you catch the worn thrust bearing before it spins in the block it is a
relatively simple operation of replacing bearings. If it spins in the block and
you are lucky it only wears away the rear thrust face in the block which can be
repaired by welding and refinished by hand or by fixing semicircular steel shims
with countersunk machine screws into tapped holes in the block.
If the bearing spins and damages the radial faces as well as the thrust faces
the block is probably toast unless you can find someone prepared to build up the
radial faces by welding and then line bore just the thrust bearing back to match
all the others.
From your description in your email to me (which is kind of personal - I could
describe the same noises and stuff in a completely different way which would
mean something completely different to someone else) I think your next step is
to get underneath, undo the front clamp on the drive shaft and measure the
end float on the crank. This is the only way to put your mind at rest (or confirm
the worst). Sorry.
But if you catch it early it is just a set of bearings and a new Oil pan gasket.
Jon in NZ
======
Thrust bearing failure
Date Posted
11/14/2001
Posted By
"Jim Bailey"
Message:
failure of the main bearing thrust bearing is very rare in a 5 speed , the
drive shaft slides freely through the center of the clutch discs . Which
means it cannot push the crankshaft forward or pull back overloading the
side flanges of the bearing . The only time it is under load is when the
clutch pedal is depressed and the clutch arm is pulling the release / throw out
bearing back away from the engine . This pulls the pressure plate
fingers away from the friction discs allowing the flywheel and pressure plate to
continue rotating but the disc (or discs) and drive shaft and 5th
gear stop rotating . Which is just another reason not to wait at a stop light
with the car in gear holding the clutch to the floor .. The release
bearing does not like it , the pressure plate does not like it , the motor
thrust bearing does not like it and if the rubber clutch hose should burst
.. You just " popped the clutch" and away you can go.. But back to the thrust
bearing .(very simplified) The surface of the main bearing where the
crankshaft passes through is like a metal arm band around your wrist only your
wrist is spinning at 6,000 RPM . Without oil the softer bearing
surface metals quickly overheat and erode , continued and it scratches and
begins overheating the crankshaft do it long enough and the crankshaft may
crack . The damages increases the clearance between the bearing and the crank
and internal knocking noises may be heard (failed rod bearing really
knock but a higher lighter knock ). Normally oil pressure creates a film of oil
between the crankshaft and the bearing and the metal never touches . As
it turns a small wave of oil is pushed along the bearing surface . The sides
flanges of the thrust bearing (the part which keeps the crank from sliding
back and forth) do not push a oil wave around but the spinning crank acts like a
potters wheel and wants to sling the oil away from the surface which
is one reason it cannot withstand much pressure front to back but can handle the
pounding of all the explosions for thousands of miles. The typical
indication of failed thrust bearings will be that the engine will not turn over
when hot ,the starter motor is not strong enough to overcome the drag
or when changing oil the oil looks like silver metallic paint from all the
ground up metal . It can be checked by measuring how much endplay you
observe , measure when prying the crank shaft back and forth .
Jim Bailey
928 International
Jim@928intl.com
www.928intl.com
(714)632-9288
=========
From: <Rickythorpe at aol.com>
To: <928uk at 928.org.uk>
Sent: Thursday, October 09, 2003 3:22 PM
Subject: [928uk] re seizing engine
when my 1985 S2 does approx 20 miles and gets hot it stalls you then wait 10 to
15 minutes and it will start again but wont idle it is also very sluggish to
start. a garage has changed the air flow meter, battery and repaired the starter
motor [apparently it was faulty] i still have the same problem and they are now
telling me it is the thrust washers around the crankshaft possibly but they can't
be certain, all this from a main Porsche dealer, ouch! any ideas please.
===========
From: 928uk-bounces at 928.org.uk [mailto:928uk-bounces at 928.org.uk]On
Behalf Of Paul R Smith
Sent: 09 October 2003 16:35
To: Rickythorpe at aol.com; 928uk at 928.org.uk
Subject: Re: [928uk] re seizing engine
I assume your car is an auto?
IF it is the crankshaft thrust bearings, this is very serious. It is almost
always caused by the flex plate becoming pre-loaded.
What you must do is stop driving it, take off the lower bellhousing (6 bolts),
then slacken the big allen screw clamping the flex-plate to the drive shaft, and
check how far the clamp moves (this indicates how much the flex-plate was
pre-loaded).
Then you need to move the crankshaft forwards and backwards and measure how far
it moves. This will indicate how worn the thrust bearings are.
If they are worn beyond the wear limit ( 0.4 mm ), its an engine out job to
change. If the wear is much greater (say 2 mm ) there is a serious danger that
the crankshaft is scrap. If you continue to drive it, you will also scrap the
block, and risk it seizing solid.
If you're not up to checking it yourself, get it looked at by someone who can.
Ignoring this will be fatal to your engine.
Smiffy
===========
Ricky
The symptoms you describe are the classic symptoms of impending thrust bearing
failure due to preload of the flex plate. Amplifying what Smiffy has
said; If the Engine is not making knocking noises then there is a chance that
the centre main thrust bearing has not spun in it's housing (Yet !). If
this has not happened then you may find that there is no critical crank damage.
Yes the counterweights on the crank may have been wearing against
the block but this in itself is not critical.
As smiffy says you need to check how much end float there is. and relieve the
preload that has built up. You may also find that the engine does start
knocking after you have relieved the preload. From your description it does
sound bad enough that you will have to pull the engine and change the thrust
bearing as a minimum. I hope that the bearing has not spun. Once you go down the
route of pulling the engine there is a whole list of other stuff you may
as well do wyait.
I have a series of pics of what happens when the bearing spins I could send you
if you like but they are about 10Mb worth.
Regards
Jon
======
Theo
looking at the specs - max end float when new is 0.3mm, max worn is 0.4mm so if
the end float is greater than 0.4mm I would say that you have to pull the engine
(Realistically you can't do the job with the engine in the car) and replace the
thrust bearing.
Realistically if there are white metal particles in the filter you have to pull
the engine, as there is some form of bottom end bearing wear.
If you catch the worn thrust bearing before it spins in the block it is a
relatively simple operation of replacing bearings. If it spins in the block and
you are lucky it only wears away the rear thrust face in the block which can be
repaired by welding and refinished by hand or by fixing semicircular steel shims
with countersunk machine screws into tapped holes in the block.
If the bearing spins and damages the radial faces as well as the thrust faces
the block is probably toast unless you can find someone prepared to build up the
radial faces by welding and then line bore just the thrust bearing back to match
all the others.
From your description in your email to me (which is kind of personal - I could
describe the same noises and stuff in a completely different way which would
mean something completely different to someone else) I think your next step is
to get underneath, undo the front clamp on the drive shaft and measure the
endfloat on the crank. This is the only way to put your mind at rest (or confirm
the worst). Sorry.
But if you catch it early it is just a set of bearings and a new OIl pan gasket.
Jon in NZ
======
Ok, for all of you wondering what my query was all about, here's the what and
why:
My car suddenly started to sound different after a long drive, hollow,
resonating, and vibrated a lot at 2000-3000 rpm. Its never been like this
before. I feared a TBF as some of the symptoms seemed to match and all other
things I checked seemed ok. Idle was ok, and the symptoms did not change warm or
cold.
I went to see an independent Porsche mechanic who is very 928 experienced, and
asked for his advice. He almost immediately said that part of the sound was from
the motor mounts being collapsed. I knew the engine was low in the bay but did
not expect it to cause this. He also said to replace oil and filter, and
carefully inspect oil and filter for particles. If none exist, just put in a
good oil and it will be ok.
I did. Found no particles or signs of wear in the filter. None. But a few
particles in the oil that looked like sand. Strange. Could not find any in the
filter. Definitely not looking like metal. (Puzzled, I am)
After filling up with oil I ran the engine for a few mins and it seemed more
quiet. I did a test-drive and the difference is substantial. The vibrations are
still there more or less (mounts) but the engine sounds like it used to do. I
tested at some speed and accelerated a few times. All seems back to normal.
Let's hope it was all false alarm.
The lifters are also a lot less noisy. I immediately noticed this change.
So........ my lessons:
1. don't get worried and suspect engine failure right away
2. I do need to put in those new mounts I have
3. Oil does degrade and needs to be of good quality
Oh, and for those of you believing people saying you only need to add oil: I
don't believe this. I replace oil and filter at least once a year or at 20.000
km and use a new filter. I'm actually driving just 10.000 km a year or so. Oil
"wears out" and gets contaminated!
regards,
--
Theo
1988 928s4 cherry red
The Netherlands
http://928s4.no-ip.info